Medical articles are frequently used for delivery of therapeutic agents. For example, an implantable or insertable medical device, such as a stent or catheter, may be provided with a polymer matrix coating layer that contains a therapeutic agent. Once the medical device is placed at a desired location within a patient, the therapeutic agent is released from the polymer matrix and into the patient, thereby achieving a desired therapeutic outcome.
Techniques for changing the release rate of the therapeutic agent from the polymer matrix coating layer include the following: (a) changing the therapeutic agent loading level, (b) adding one or more additional polymers to the polymer matrix, for example, to alter the hydrophilic/hydrophobic balance of the matrix, (c) disposing one or more polymeric barrier layers over the therapeutic-agent-containing region, and (d) where a biodegradable polymer matrix is employed, changing the degradation properties of the polymer.
As is well known, silicates such as clay belong to a family of minerals that have a layered structure. The atoms within a given layer are tightly bound together, but the forces between adjacent layers are relatively weak. As a result, it is possible to separate the layers form one another. Under proper conditions, a single atomic layer can be separated from neighboring layers. Consequently, preparations are presently commercially available which contain molecularly thin platelet particles. These platelet particles are typically on the order of about 10 Angstroms (or 1 nm) in thickness, and on the order of 0.1 to 10 microns in lateral dimension. The use of such platelet particles to reduce the permeability of polymers has been described in U.S. Pat. No. 6,232,389, with regard to coated elastomeric articles, such as sports balls and other inflatable articles.